(a) Field of the Invention
This invention relates to drive belt pulleys and belt drive systems.
(b) Description of the Related Art
In drive systems using a flat belt, the flat belt when running often causes wobbling and sidetracking towards one side of the pulley. This is because a flat belt is susceptible, as compared with the other kinds of drive belts, to position changes of drive system components, such as deviation of a pulley shaft from its normal position, deflection thereof due to change in radial load thereon, and pulley wobbling. If such a belt wobbling or sidetracking occurs, then the flat belt may come into contact with a flange of the flat pulley, resulting in a fuzzed flat belt side face or a frayed cord.
A known approach to this problem is to crown the outer periphery of a flat pulley (i.e., to form it into a convex surface). There is also a proposed technique to form the crown at the outer periphery of a pulley into the shape of a sphere around the rotation center of the pulley (see, for example, Japanese Unexamined Utility-Model Publication No. 59-45351). This proposed technique is intended, when a tension difference arises between the right and left portions of the flat belt to cause an inclination of the pulley shaft and its attendant deviation of the flat belt to one side of the pulley, to take the advantage of rotation moment acting on the pulley by tension of the flat belt to eliminate the inclination (angular deviation) of the pulley shaft and the deviation of the flat belt.
There is also known a flat pulley the outer periphery of which has a multiplicity of grooves formed at regular intervals along its circumference (see, for example, Japanese Unexamined Patent Publication No. 6-307521). Each groove extends to form a V-shape symmetrically from the middle of the pulley width toward both sides. These grooves create between the flat belt and the pulley a frictional force which allows the flat belt to approach the middle of the pulley width, thereby preventing wobbling and sidetracking of the belt.
There is still also known a technique to place guide rollers to both sides of a flat belt and thereby limit the running position of the flat belt (see, for example, Japanese Examined Utility-Model Publication No. 63-6520).
The first-mentioned approach of crowning the pulley groove surface, however, has the following problem. When the curvature of the crown is set at a small value taking count of running stability of the flat belt (prevention of wobbling and sidetracking), stress is focused on the middle of the belt width. This prevents effective utilization of the entire belt width for power transmission and leads to early cord fatigue and degraded power transmission performance.
The above-mentioned technique of forming the pulley crown into a sphere around the rotation center of the pulley, even if it enhances the effect of preventing sidetracking of a drive belt, still has the above problem that stress is focused on the middle of the belt width by the pulley crown.
The above-described grooving of the flat pulley surface raises the production cost of the flat pulley. In addition, it is difficult to certainly prevent wobbling and sidetracking of the flat belt simply by the grooving.
When the approach is employed of limiting the running position of the flat belt by placing guide rollers or the like to both sides thereof, both sides of the flat belt always contact such a limiting member. As a result, the belt side face is likely to fuzz and the cord is likely to fray. This creates the need for applying to the flat belt a special process for preventing such adverse effects, which is a disadvantage to reduction of production cost of the flat belt.
For the above-described reasons, the fact is that though flat belt drive systems have lower loss due to belt flexing and very higher power transmission efficiency as compared with systems using other types of belts, such as V-belts, they are not exploited enough.